1. Field of Invention
This invention relates generally to inertial sensors and, more particularly, to the packaging of such sensors.
2. Related Art
In order to function properly, inertial sensing elements commonly require a controlled environment such as very low pressure or vacuum, low humidity, and/or a specific gas. In order to maintain such environments over the life of a sensor, it is usually necessary to hermetically enclose the sensing element within a package or cavity. In addition, electronics associated with the sensing element must also be protected from moisture, which typically requires a separate package or enclosure or over-molding with a moisture resistant compound.
In sensors of the prior art, the sensing elements are often individually enclosed within packages of their own. These packages can be of the same material as the sensing elements, as, for example, in micromachined silicon devices that include hermetically sealed caps. Alternatively, the sensing element can be sealed in a package made of a different material, e.g. a quartz or silicon tuning fork mounted in a sealed ceramic package.
The sensing elements and associated electronics, such as integrated circuits and passive components, are often mounted side-by-side on a substrate. This increases the necessary surface area of the substrate, which increases cost and size of the sensor. It also complicates routing of traces to connect the sensing elements and electronic components.
Moreover, depending on how the components are arranged, separate testing of the individual components of a sensor can be difficult or impossible. When the components cannot be tested separately, it may be necessary to reject an entire assembly even if only one of the components is faulty, and scrapping functional components along with the bad adds unnecessarily to the cost of the product. This difficulty arises even when sensing elements and electronics are mounted in a vertical arrangement such as a tuning fork positioned above an integrated circuit in a cavity. If there is a problem with the sensing element, the relatively costly integrated circuit is scrapped as well.
Over-molding compounds, such as silicone gel or plastic, are commonly used to protect the sensing element, electronic elements, and interconnections from moisture. The materials and processes involved in over-molding introduce additional cost, complexity, and the potential for additional failure modes, and the over-molding is done in addition to any hermetic seals enclosing the individual sensing elements.